Method of manufacture and composition for magnetic cores



United States Patent Office Patented Mar. 3, 1970 METHOD OF MANUFACTURE AND COMPOSITION FOR MAGNETIC CORES Albert N. Copp, State College, Pa., assignor to Western I ABSTRACT OF THE DISCLOSURE A method of and a new composition of matter for use in manufacturing magnetic powder cores, particularly of the Permalloy type. Finally divided metal particles of magnetic material are insulated in a slurry consisting esse'ntially of sodium silicate, magnesium hydroxide, aluminum silicate, magnesium silicate and a viscosity modifying agent selected from the group consisting of polyvinyl alcohol and carboxymethyl-cellulose. A peculiar percentage relationship, by Weight, between the aluminum silicate and magnesium silicate is primarily responsible for magnetic cores made from the composition exhibiting consistently high, linear positive temperature coefficients and relatively high permeabilities. The particular types of viscosity modifying agents selectively employed make possible a one-coat application of the insulating materials on the magnetic particles, and effect a substantial reduction in the total losses normally experienced in cores having aluminum silicate as an insulating material.

This invention relates to a method of and a new composition of matter for use in manufacturing magnetic cores and, more particularly, to a method of and a composition for use in manufacturing Permalloy type magnetic powder cores which exhibit certain improved physical and magnetic properties. It is an object of this invention to provide a new and improved magnetic powder core of such character.

Certain tuned or balanced inductance-capacitance (LC) networks utilize as the inductance element thereof a coil wound on a magnetic core of the Permalloy type, and as the capacitance element thereof a capacitor having polystyrene plastic as the dielectric medium. In order to insure that the resonant frequency of such networks remain within prescribed limits with respect to changes in temperature, it has been found necessary to balance or offset the negative and substantially linear rate of change of capacitance with temperature characteristic exhibited by the polystyrene capacitors with the positive permeability versus temperature counterpart exhibited by the Permalloy type inductors. Hereinafter, a change of permeability with temperature of an element will be referred to simply as the temperature coeflicient of that element.

Prior art magnetic core compositions utilizing finely divided dust particles of a nickel-iron alloy, referred to as Permalloy, have generally proven unsatisfactory with respect to yielding cores which have a positive, linear temperature coeflicient closely approximating, in absolute value, the negative, substantially linear temperature coefiicient of polystyrene capacitors.

Failure to achieve the desired characteristics heretofore is believed to reside, in large part, on the differential thermal expansions of the magnetic particles and the insulating coatings thereon. More particularly, the insulating coatings or films on and separating adjacent particles in a powder core effectively establish and act as air gaps in a magnetic circuit and, as such, have a direct bearing on core inductance or permeability. In addition, variations in the spacing of these air gaps, resulting from the dilferential thermal expansions of the particles and coatings, are believed to be the primary cause of changes in core permeability (or inductance)'with temperature, which defines the temperature coefficient.

It has been found that the temperature coefiicient af magnetic powder cores made from a given lot of Permalloy powder can be increased by reducing the thermal coefiicient of expansion of the insulating coating and vice versa. For example, in two well known insulation forming compositions utilized extensively heretofore, a refractory metal silicate in the form of either talc (magnesium silicate) or colloidal clay (aluminum silicate), has been utilized together with magnesium hydroxide and an alkali metal silicate, such as sodium silicate. As between colloidal clay and talc, the former has a lower coefficient of thermal expansion than talc, and therefore, the substitution of colloidal clay for talc has been found to result in a reduction of the coefficient of thermal expansion of the insulating coating on the magnetic particles, and an increase in the temperature coefficient of the finalized cores.

Unfortunately, however, the mere interchange of colloidal clay for talc in an attempt to obtain a high particular average value for the temperature coefiicient of the resultant cores does not insure the attainment of consistently high and linear temperature coefficients. Moreover, the selective use of either talc or colloidal clay with sodium silicate and magnesium hydroxide, for example, gives rise to other selective disadvantages which are not necessarily directly related to the temperature coefiicient characteristics desired.

In order to amplify further on the above problems, reference is made first to one well known Permalloy core-forming composition utilizing talc, genrally referred to as the Bandur composition, disclosed in US. Patent 2,105,070 of A. F. Bandur, issued Jan. 11, 1938. Magnetic cores manufactured in accordance with that teaching exhibit satisfactory overall magnetic and physical characteristics for many general applications. However, with respect to certain specific circuit application, such as where it is important that an inductance element exhibit a high, linear temperature coeflicient, Bandur type powder cores have not proven satisfactory as they normally exhibit an average temperature coefficient, expressed in change of permeability in parts per million per degree Fahrenheit, of +40 p.p.rn./ F., :20 p.p.m./ F., from room temperature (70-80 F.) to about 140 F. This range for the positive temperature coeflicient is, on the average, far below the nominal negative temperature coefficient of p.prn./ R, which typically characterizes polystyrene capacitors. In addition, the temperature coeflicient often does not vary linearly so as to match the negative counterpart exhibited by such capacitors.

Aside from the resulting temperature coefiicient characteristics, the Bandur composition also normally requires a four-coat operation to provide an adequate insulation build-up on the magnetic particles, the denseness and uniformity of this coating having a significant effect on core losses.

With respect to permalloy core forming compositions utilizing colloidal clay rather than talc as one of the insulating materials, the magnetic cores resulting therefrom may possibly attain a temperature coeflicient of +75 p.p.m./ F., however, such results have proven to be very unpredictable, as the temperature coefiicient has been found to vary considerably from one lot to the next, and to vary non-linearly, as is often the case when tale is used as an insulating constituent.

Moreover, cores manufactured with colloidal clay generally exhibit relatively high core losses, often in excess of 0.240 (ohms/henry/unit of permeability). In addition, the utilization of colloidal clay, as with talc, normally requires several applications of insulating materials to the magnetic particles in order to provide a satisfactory coating thereon.

Because of the aforementioned problems encountered heretofore in manufacturing Permalloy type magnetic powder cores with consistently high and linear temperature coefficients, it has generally been necessary in certain demanding applications to test and categorize individual cores made from prior art compositions so that only those cores which had a temperature coefficient falling within an acceptable range, such as from +55 n.p.m./ F. to +95 p.p.m./ F, would be adequately matched with polystyrene capacitors having an average temperature coefiicient of 75 p.p.m./ F. Of course, the necessity of having to select individual cores from a large number of cores is not only a time consuming and costly operation, but results in usable core yields of considerably less than 100 percent.

Therefore, it is another object of this invention to provide an improved method and a new composition of matter conducive to the manufacture of magnetic powder cores which exhibit an average, linear temperature :oefficient of +75 p.p.m./ F., :20 p.p.m./ F.

It is a further object of this invention to provide an improved method and a new composition conducive to the manufacture of magnetic powder cores wherein only one coating application of insulating materials is normally required to achieve a dense, uniform and durable insulating coating on the magetic particles, and which results in physically strong cores exhibiting relatively low core losses.

It is an additional object of this invention to provide improved method and a new composition conducive to the manufacture of Permalloy-type magnetic powder :ores which are particularly suitable for utilization with polystyrene capacitors as matched circuit elements of a tuned LC network.

It is still another object of this invention to provide a. new and improved Permalloy-type magnetic powder :ore which not only exhibits a consistently high, linear temperature, coefiicient, but also exhibits overall magnetic and phyical characteristics comparable with those realized with prior art cores of similar type.

In accordance with the principles of this invention, a. new magnetic core forming composition is formed ini- ;ially as a slurry comprised of finely divided metal par- :icles of the permalloy type, and insulation-forming maierials comprising an alkali metal silicate, magnesium lydroxide, two different refractory metal silicates, water and a viscosity modifying agent. The basic form of the :omposition comprises the dry magnetic powder resulting from the heating of the slurry to evaporate the vola- :ile constituents therein, and the magnetic cores of the present invention comprise the pressed and fired reaction product of the dry magnetic powder.

Considering the various core-forming constituents rnore specifically, the metal particles consist of a nickel- .ron-molybdenum alloy, and the insulation forming ma- :erials comprise predetermined quantities of sodium silizate, magnesium hydroxide, colloidal clay (aluminum ;ilicate), talc (magnesium silicate), water, and a viscosity modifying agent selected from the group consisting of polyvinyl alcohol and carboxymethyl-cellulose.

The viscosity modifying agent, added to the core form ing composition in accordance with the principles of this invention, is particularly significant in that it has been Found to improve the degree of suspension of the finely livided metalparticlesand the insulating materials there- For within an agitated water slurry. As a result, when the ;lurry is subjected to heat during a drying operation car- 'ied out thereon, the insulating materials have been found be more thoroughly and uniformly coated on the netal particles than was possible with prior core form- .ng compositions. One possible explanation for this from 1 more technical standpoint is that the addition of the viscosity modifying agent is believed to defiocculate (separate) the clay particles, and thereby improve the ultimate bonds between the particles and their respective insulating coatings.

Because of the degree of uniformity with which the insulating materials adhere to the metal particles, it has been found that the resultant dry composition may be molded into cores without the need of repeated applications of the insulating materials to the initially coated particles, as is generally necessary when prior permalloy core forming compositions are employed. The utilization of a one coat insulating process, of course, greatly reduces the time and costs involved in the manufacture of magnetic powder cores.

An additional advantage realized through the use of the viscosity modifying agent is that it reduces substantially the total losses normally experienced in cores having colloidal clay rather than talc as one of the insulating materials.

The two refractory metal silicates, namely, the colloidal clay and talc, when utilized within the percentage ranges set forth hereinafter, have been found to result in permalloy type magnetic powder cores which consistently exhibit higher average, linear temperature coefficients than have been realized heretofore. In addition, the temperature coefficients of the present cores are also more predictably controlled and fall within a more restrictive range, in terms of the percent deviation from the average value.

More specifically, it has been foundthat the present cores typically exhibit an average, linear temperature coefiicient of +75 p.pm./ F., :20 p.p.m. F., as compared to an average TC of +40 p.p.m./ F., :20 p.p.m./ F., realized in accordance with other prior art cores, such as those manufactured in accordance with the aforementioned Bandur composition. The interrelationship between the thermal coefficients of expansion between the talc, clay and Permalloy particles, for the percentage ranges, by weight, of each in the core forming compositions set forth hereinafter, is believed to contribute significantly to the beneficial core characteristics ultimately obtained.

Cores exhibiting the higher average temperature coefiicient of +75 p.p.m./ F. are, of course, particularly important when intended for use in compensating for or balancing out a substantially equivalent, but negative temperature coefficient exhibited by polystyrene capacitors in LC electrical networks.

Additional advantageous characteristics realized in accordance with the principles of this invention are that the core forming composition, while still in a slurry state, is more conducive to spray drying as well as rotating drum drying, by reason of the presence of the viscosity modifying agent, and the cores are generally stronger in both the green and fired states, and normally exhibit lower losses than prior cores of similar type, by reason of the new and improved overall composition.

Permalloy type powder cores made according to this invention, of course, are not limited to demanding temperature coefiicient applications, as they also exhibit other overall physical and magnetic properties which are comparable to those of cores made by the Bandur process, as well as by others.

This invention, together with further objects and advantages thereof, will best be understood by reference to the following description which sets forth in detail both preferred ranges for the core forming constituents, and a brief description of the method by which the composition is transformed into a new and improved magnetic powder core of the permalloy type.

The present composition is formed by mixing with each 1000 grams of finely divided Permalloy-forming metal particles, which consist .of 80.0 to 83.0% by weight nickel, 15.0 to 17.0% by weight iron, and 1.75 and 2.45% by weight molybdenum, the following constituents for and ranges of the insulation-forming materials: 4.93 to 11.41 grams of sodium silicate wherein the sodium oxide (Na O) to silicon dioxide (SiO ratio is 2.84 to l; 6.72 to 15.48 grams of magnesium hydroxide liquid containing 32 to 40 grains of Mg(OH) per fluid ounce; 5.24 to 12.05

6 barrel, 10,000 cc. of distilled water is measured poured into a mixing kettle. Then 920 gramsof sodium silicate having an Na O to SiO ratio of 2.84 t 1.0 is Weighed out and added to the water in the mixing kettle. A mixer associated with the kettle is started and while the gut and grams of colloidal clay (aluminum silicate); and 0.62 to contents of the kettle are being agitated, 1250 grams of 1.41 grams of talc (magnesium silicate). To this mixture magnesium hydroxide liquid containing 3240 g ains of is added 66.0 to 110.2 cc. of water, preferably distilled, Mg(OH) per fluid ounce is added to the contents of the and 14.89 to 24.81 cc. .of a viscosity modifying agent kettle. I selected from the group consisting of a 10% by weight 10 After five additional minutes of agitation of the consolution of polyvinyl alcohol and a 0.5% by Weight solutents in the mixing kettle, the contents therein are transtion of carboxymethylcellulose. ferred to the mixing barrel. At this time, 2250\cc. of After all of these constituents are mixed sufiiciently to either a 10% by weight solution of polyvinyl alcohol or form a homogenous slurry, heat is applied to the slurry a 0.5% by weight solution of carboxymethyl-ce lulose, to evaporate the volatile constituents or components comprising the viscosity modifying agent utilized in actherein, whereby a dry, insulated Permalloy powder is cordance with the principles of the present inven ion, is obtained. This powder is then pressed into cores, with also added to the mixing barrel. All of the individu lconthe cores thereafter being fired to establish a stabilized st tllents Which ultimately form one preferred ne w core magnetic state. The resultant insulating coating on the forming composition are w in the rr l, and it i permalloy particles of the cores is thus seen to comprise rotated until the contents thereof form a hOITl geIlOuS the fired reaction product of the heat treated slurry, i.e., miXtllrethe fired reaction product of the dry magnetic powder. As a final step in preparing the composition fo r For the percentage ranges of the core-forming constituents ti cores h t i li d to h i i b l, for set forth above, the resultant insulating coating will l by a gas fl di d i t h id h f, ypi lly l Within a range of 0 7 by W ight, This heat evaporates the volatile constituents in t e barof the total y Weight of the insulated Particles, with a rel whereby a dry, insulated Permalloy powder is obtained range of 1.3 to 1.8% generally being preferred. which is suitable for pressing into core rings for subse- After being fired, the cores exhibit a permeability in quent firing to make finished magnetic cores. More spethe range of 115-145, a total core loss less than 0.240 cifically, the constituents are heated in the mixing barrel (ohm/henry/unit of permeability), typically about 0.160 until such time as the non-volatilized constituents each a unit, and a relatively high average temperature coefficient temperature of approximately 260 F. When the preof approximately +75 p.p.m./ F., $20 p.p.m./ F., scribed temperature is reached, the resultant dry, insulated which is linear. The values for permeability and core loss powder is sifted through an 80 mesh screen and c llected are both referenced to a core subjected to a frequency of in individual lots. 1800 cycles per second, and a flux density of 20 gauss. In order to produce finished permalloy cores fr om the By way of further illustration, one preferred method dry magnetic powder, the following process steps are and specific composition, for manufacturing Permalloy performed. The magnetic powder is formed or pressed type powder cores of the present invention in corninto toroidal shaped cores by charging measured piortions mercially significant quantities will now be described. of the powder in a mold cavity of a molding press, for First 250 lbs. (113,400 grams) of finely divided permalloy example, and then subjecting the powder to a p es sure metal particles, prepared in accordance with the general in the neighborhood of 200,000 lbs. per square inch. Durteaching of the aforementioned Bandur patent, are careing the application of this pressure, as is well known, fully weighed out and charged into a mixing barrel. The the metal particles are subjected to strains which may finely divided particles, which pass through a 120 mesh impair the magnetic properties of the core. Therefore, seive, preferably consist, by weight, of 80-83% nickel, the cores are annealed by a heat treatment, prefer ably in 15-47% iron and 1.75-2.45% molybdenum. a hydrogen atmosphere, at a temperature from 1000 to The mixing barrel into which the particles are charged 1200 F., for a period of approximately 20 minute Duris mounted for rotation about an axis preferably inclined ing this heat treatment, the insulting materials ar fully approximately 35 with respect to the horizontal. A stacured. tionary scraper positioned near the top of the barrel is A preferred method and one specific example of the also advantageous in insuring thorough mixing of the composition has thus been set forth which resul s in a contents in the :barrel when it is rotated. new and improved Permalloy powder core wher in the Once the mixing barrel is charged with the permalloy metal particles thereof acquire insulating coatings of particles, 874 grams of colloidal clay (aluminum silicate, about 1.6%, by weight, of the total dry weightlof the Al O -2SiO -2H O) and 114 grams of talc (magnesium insulated particles. Other preferred examples of the core silicate, 4M O-5SiO -H O) are carefully weighed out and forming composition for different percentages, by bveight, added to the barrel. The barrel is then rotated for a of the insulating materials coated on the particles are set period of approximately 3 minutes to facilitate the dry forth in the following table, wherein the constituen quanmixing of the colloidal clay, the talc and the permalloy tities associated with each example are referenced to 250 particles. The order in which the above constituents are lbs. of permalloy particles as a starting base.

Polyvinyl alcohol or Sodium Magnesium carboxy- Percent silicate, hydroxide, Colloidal methyl- Dlstuled insulation grams grams clay, grams Talc, grams cellulose, cc. ater, cc.

added to form the composition of matter of this invention is not critical, and the one specified is deemed to be the most desirable in accordance with the standards of good mixing practice.

It should be noted that the sodium silicate and magnesium hydroxide, as listed in the above table, are in liquid form, and that-the percentages of insulation listed are calculated on the basis of the weight of solid ma- As the above constituents are being dry mixed in the terials in a dry state. It should also be noted that a 0.5%

by weight solution of carboxymethyl-cellulose may be substituted for the polyvinyl alcohol utilized in the specific :xample described above.

While specific quantities have been set forth in the above table for the various core forming constituents of :ach example, these constituents may be varied within plus )r minus 25 percent of the specified amounts without ;ignificantly changing the magnetic properties of the par- :icular core produced therefrom. For the sake of clarity, :ertain of the appended claims have been drafted such :hat the compositions claimed therein have, as a starting Jase, 1000 grams of magnetic particles. Accordingly, the iumbers utilized in the above table are equated to this [000 grams base by simply using 113.4 as a common livisor.

There has thus been disclosed herein an improved nethod and new and improved compositions for producng Permalloy type magnetic powder cores which are ahysically strong, and exhibit both predictably high, linear :emperature coefiicients of +75 p.p.m./ F., :20 p.p.m./ F., and low core losses, with all of these attributes being 'ealized with a one coat layer of insulation on the metal particles.

What is claimed is:

1. In a method of making a composition of matter vherein finely divided magnetic metal particles are given in insulating coating, including as materials thereof, nitially mixed in a slurry, an alkali metal silicate, magiesium hydroxide, magnesium silicate and water; and wherein the slurry is subjected to a heat treatment to :vaporate .the volatile constituents of the slurry to form 1 dry, magnetic powder, the improvement comprising:

adding aluminum silicate in the range of approximately 3.7 to 19.2 times the amount of magnesium silicate, by weight, and based on 1,000 grams of said metal particles as a unit of reference, 14.89 to 24.81 cc. of a viscosity modifying agent selected from the group consisting of a 10%, by weight, solution of polyvinyl alcohol and a 0.5%, by weight, solution of carboxymethyl-cellulose to the slurry prior to the heating step.

2. In a method of making a composition of matter in accordance with claim 1, the alkali metal silicate comarises sodium silicate.

3. In a method of making a composition of matter in accordance with claim 2, said finely divided magnetic netal particles are given an insulating coating of between ).975 and 2.25%, by weight, of the total dry weight of the 'esultant insulated particles, and based on 1000 grams of he metal particles as a unit of reference, the insulating :oating consisting essentially of the heat-treated prodrct of:

4.93 to 11.41 grams of the sodium silicate having a N3 to Si0 ratio of 2.84 to 1;

6.72 to 15.48 grams of the magnesium hydroxide in liquid form containing 32 to 40 grains of Mg(OH) per fluid ounce; I

5.24 to 12.05 grams of the aluminum silicate;

0.62 to 1.41 grams of the magnesium silicate;

14.89 to 24.81 cc. of the viscosity modifying agent selected from the group consisting of a by weight solution of polyvinyl alcohol and a 0.5% by weight solution of carboxymethyl-cellulose; and

66.00 to 110.2 cc. of water.

4. A method of manufacturing Permalloy-type magnetic :owder cores exhibiting consistently high, linear temperavure coefiicients and low core losses comprising the steps )f:

mixing finely divided Permalloy metal particles together with an insulation-forming Wet mixture of sodium silicate, magnesium hydroxide, aluminum silicate, magnesium silicate and water in selected proportions suflicient both to establish the consistency of a slurry when agitated and to produce an ultimate insulating coating on said metal particles of 0.975 to 2.25%, by

8 weight, of the resultant total dry weight of the insulated particles;

adding to and mixing with said Permalloy particles and said insulation forming wet mixture, and based on 1,000 grams of said metal particles as a unit of reference 14.89 to 24.81 cc. of a viscosity modifying agent selected from the group consisting of a 10%, by weight, solution of polyvinyl alcohol and a 0.5%, by Weight, solution of carboxymethyl-cellulose, said viscosity modifying agent improving the degree of suspension of the Permalloy particles and the insulation-forming materials in the slurry, and ultimately minimizing the core losses, and

heating said mixed slurry with the viscosity modifying agent sufliciently to evaporate the volatile components in said slurry and to form a Permalloy core-forming powder composition;

charging the resultant dry composition into a mold and applying suflicient pressure thereto to form a Permalloy core; and

applying heat to said formed Permalloy core both to cure and to reduce any physical strains therein which could otherwise adversely affect the magnetic properties thereof.

5. A method in accordance with claim 4 wherein said Permalloy particles consist, by weight, of 80.0 to 83.0% nickel, 17.0 to 15.0% iron and 2.45 to 1.75% molybdenum, and wherein for each one thousand grams of said Permalloy-forming particles, said insulation forming wet mixture added and mixed therewith consisting essentially of 4.93 to 11.41 grams of sodium silicate having a Na O to SiO ratio of 2.84 to 1, 6.72 to 15.48 grams of magnesium hydroxide liquid containing 32 to 40 grains of Mg(OH) per fluid ounce, 5.24 to 12.05 grams of aluminum silicate, 0.62 to 1.41 grams of magnesium silicate, 66.00 to 110.2 cc. of water, and wherein said viscosity modifying agent consists of 14.89 to 24.81 cc. of a 10% by weight solution of polyvinyl alcohol and a 0.5% by weight solution of carboxymethyl-cellulose.

6. A method in accordance with claim 5 wherein the nonvolatilized constituents of the mixed slurry are heated to a temperature of approximately 260 F. to evaporate the volatile constituents, wherein the dry powder composition is subjected to a pressure of approximately 200,000 lbs./ square inch in the mold to form said cores and Wherein the formed cores are finally subjected to a temperature between 1000 to 1200" F. for a period of approximately 20 minutes to cure said molded cores.

7. A method of manufacturing Permalloy-type magnetic powder cores comprising the steps of:

dry mixing for a period of 1 to 5 minutes in a suitable container one thousand grams of finely divided Permalloy metal particles consisting, by weight, of 80.0 to 83.0% nickel, 17.0 to 15.0% iron and 2.45 to 1.75 molybdenum, with 6.99 to 9.64 grams of aluminum silicate, and 0.82 to 1.13 grams of magnesium silicate;

wet mixing in a second suitable container 6.58 to 9.13

grams of sodium silicate having a Na O to SiO ratio of 2.84 to 1, 8.96 to 12.39 grams of magnesium hydroxide liquid containing 32 to 40 grains of Mg(OH) per fluid ounce, 66.00 to 110.2 cc. of water and 14.89 to 24.81 cc. of a viscosity modifying agent selected from the group consisting of a 10% by weight solu tion of polyvinyl alcohol and a 0.5% by weight solution of carboxymethyl-cellulose, said mixing being for a period of time sufficient to obtain a homogenous mixture thereof;

adding the homogenous wet mix constituents in the second container to the dry mix constituents in said first mixing container;

mixing said dry mix constituents with said wet mix constituents until a'homogenous wet composition thereof is formed;

applying heat to said homogenous wet composition until the volatile constituents therein are evaporated;

charging the resultant dry powder composition of insulated Permalloy particles into a core-forming mold and then subjecting the composition to a pressure sufficient to form a durable magnetic powder core; and

curing and releasing any adverse physical strains in said pressed core by heating the core to a temperature within a range of 1000 to 1200 F.

8. A method in accordance with claim 7 wherein said dry mix is agitated for a period of 2-4 minutes in a suitable mixing barrel, wherein said wet mix is agitated for a period of 3-7 minutes, wherein the heat applied to said homogenous wet composition to evaporate the volatile constituents therein raises the temperature of the nonvolatilized constituents to a temperature of 260 R, wherein said dry powder composition is subjected to a pressure of approximately 200,000 lbs. per square inch in the mold to form a magnetic core, and wherein said Permalloy cores are cured at a temperature of 1000-1200 F. for a period of approximately 20 minutes.

9. A new composition of matter composed of:

finely divided metal particles consisting by weight of 80.0 to 83.0% nickel, 17.0 to 15.0% iron and 2.45 to 1.75% molybdenum and, based on 1000 grams of said powder as a unit of reference,

an insulating coating on and separating adjacent metal particles consisting essentially of the heat-treated, dry powder porduct of:

4.93 to 11.41 grams of sodium silicate having a Na O toSiO ratio of 2.84 to 1;

6.72 to 15.48 grams of magnesium hydroxide liquid containing 32 to 40 grains of Mg(OH) per fluid ounce;

5.24 to 12.05 grams of aluminum silicate;

0.62 to 1.41 grams of magnesium silicate;

14.89 to 24.81 cc. of a viscosity modifying agent selected from the group consisting of a by weight solution of polyvinyl alcohol and a 0.5% by weight solution of carboxymethyl-cellulose; and

66.00 to 110.2 cc. of water.

10. As a new article of manufacture, a magnetic powder core composed of:

finely divided metal particles separated from each other an insulating coating of between 0.975 and 2.25%, by

weight, of the total dry weight of the resultant insulated particles and, based on 1000 grams of said metal particles as a unit of reference, said insulating coating consisting essentially of the heat-treated and fired reaction product of:

4.93 to 11.41 grams of sodium silicate having a Na O to SiO ratio of 2.84 to 1;

6.72 to 15.48 grams of magnesium hydroxide liquid containing 32 to 40 grains of Mg(OH) per fluid ounce;

5.24 to 12.05 grams of aluminum silicate;

0.62 to 1.41 grams of magnesium silicate;

14.89 to 24.81 cc. of a viscosity modifying agent selected from the group consisting of a 10% by weight solution of polyvinyl alcohol and a 0.5 by weight solution of carboxymethyl-cellulose, and

66.00 to 110.2 cc. of water.

11. A magnetic powder core in accordance with claim 10 wherein said finely divided metal particles are an alloy which consist, by weight, of 80.0 to 83.0% nickel, 17.0 to 15.0% iron and 2.45 to 1.75 molybdenum.

12. As a new article of manufacture, a magnetic powder core composed of:

finely divided metal alloy particles consisting by weight of 80.0 to 83.0% nickel, 17.0 to 15.0% iron and 2.45 to 1.75 molybdenum, said particles being separated from each other by an insulating coating of between 1.3 and 1.8%, by

weight, of the total dry weight of the resultant insulated particles and, based on 1000 grams of said particles as a unit of reference, said insulating coating consisting essentially of the heat treated and fired reaction product of:

6.58 to 9.13 grams of sodium silicate having a Na O to SiO ratio of 2.84 to 1;

8.96 to 12.39 grams of magnesium hydroxide liquid containing 32 to 40 grains of Mg(OH) per fluid ounce;

6.99 to 9.64 grams of aluminum silicate;

0.82 to 1.13 grams of magnesium silicate;

14.89 to 24.81 cc. of a viscosity modifying agent selected from the group consisting of a 10% by weight solution of polyvinyl alcohol and a 0.5 by weight solution of carboxymethyl cellulose; and

66.00 to 110.2 cc. of water.

References Cited UNITED STATES PATENTS 1/1938 Bandur 148104 3/1961 Harendza-Harinxma 148-104 US. Cl. X.R. 148-104 

